1. Field of the Invention
The present invention relates to an optical disc recording/reproduction apparatus and method and in particular, to an optical disc recording/reproduction apparatus and method using two groups of lenses enabling to accurately recording and reproducing a data.
2. Description of the Prior Art
The most widely known disc as an optical disc is a CD (Compact Disc). As the CD has a large recording capacity, it is now spreading as a medium for providing a computer program and data, including CD-ROM.
As an optical disc capable of recording or reproducing a data with a higher density than this CD, a DVD (Digital Versatile Disc) has been developed and normalized.
In general, when a disc recording capacity is made to have a high density, it is necessary to make larger a numerical aperture of a pickup lens used for recording or reproducing a data. That is, in order to increase a recording density, it is necessary to reduce the size of an optical spot as much as possible. A size of an optical sport formed on an optical disc is defined as L/2NA, assuming L to be a wavelength and NA to be a numerical aperture of the lens. Consequently, in order to increase a recording density, it is necessary either decrease the wavelength or increase the numerical aperture NA of the lens.
For example, because a DVD has a higher recording density than a CD, reproduction of the DVD requires a lens numerical aperture of about 0.6 in comparison to about 0.43 required for reproduction of the CD.
It is expected that the optical disc can have a further higher density than the DVD. For this, the lens numerical aperture should be a further greater value. However, the maximum numerical aperture which can be realized by a single lens is considered to be 0.6 because of the processing accuracy.
To cope with this, U.S. Pat. No. 5,125,750 discloses a solid immersion lens (SIL) to be used in combination with an objective lens, as a two-lens group, so as to realize a greater numerical aperture.
When this two-lens group is used for realizing a high numerical aperture, there is a problem that as the light coming out of the SIL enters an optical disc with a great angle, if a distance between the two lenses is deviated from a predetermined value, a wave front aberration is caused and it becomes difficult to carry out an accurate data recording or reproduction.